1. Field of the Invention
The invention relates to non-linear transistor integrated circuits (xe2x80x9cICxe2x80x9d) with thermal stabilization.
2. Related Art
Portable electronic devices have become part of many aspects of personal, business, recreational, and other activities and tasks. As the popularity of various portable electronic devices, such as personal radio frequency (xe2x80x9cRFxe2x80x9d) communication devices like portable phones, portable televisions, and personal pagers, increases, the demand for smaller, lighter, more powerful, and more power efficient electronics also increases.
There are two broad categories of transistor circuits: strongly non-linear and weakly non-linear. Strongly non-linear transistor circuits refer to circuits where distortion of the signal waveform is caused by the limiting behavior of the transistor""s current (xe2x80x9cIxe2x80x9d) and voltage (xe2x80x9cVxe2x80x9d) characteristics. The I and V characteristics may be described with high-order power series expansion and non-continuous functions. High level or multiple-frequency excitation combined with these non-linearities may perturb the direct current (xe2x80x9cDCxe2x80x9d) operating point of the transistor. Examples include strongly driven transistors and Schottky-barrier diodes because of their exponential I and V characteristics.
In contrast, weakly non-linear transistor circuits can be analyzed using a power series expansion of their non-linear I and V characteristics. The characteristics are considered to be continuous, have continuous derivatives, and do not require more than a few terms in the power series. The non-linearities and excitation signals are weak enough that the DC operating point is not perturbed. FIG. 1 sets forth a comparison of the transfer functions of a weakly non-linear 100 versus a strongly non-linear transistor circuit 110. An example of a strongly non-linear transistor circuit is a power amplifier commonly used in portable electronics.
Amplification of electronic signals is a function performed in many modern electronics communications systems. Amplification circuitry and devices tend to generate significant heat. Unfortunately, the packaging of these devices, especially within systems that have small form factors, tends to reduce the ability of these devices to dissipate heat through convection. The space surrounding these devices has also become significantly more confined as packaging size shrinks, thus reducing the opportunity for convection currents to circulate and remove heat. The packaging of these devices comprises, in significant part, materials such as plastics that are generally lighter than metal packaging. Plastics generally tend to have a greater thermal resistance to heat transfer in comparison to metals. The opportunity for heat transfer and thereby cooling of the power amplifier via conduction may be significantly reduced with the increased usage of non-metallic materials
Some manufacturers of these devices have taken the approach of adding metal heat-sinks to their devices. Unfortunately, the effectiveness of heat-sinking diminishes as the air volume available for convection cooling decreases. Some devices are designed to withstand higher temperatures. But, as packages become smaller, the heating of adjacent devices increases, which may tend to increase heat dissipation and reduce the reliability of the devices.
Typically, heterojunction bipolar transistors (xe2x80x9cHBTxe2x80x9d) enable more efficient RF power amplification than other semiconductor devices in IC format because of the high power density and the high breakdown voltage of the HBT devices. For high power designs, a multitude of matched HBTs are connected in parallel forming a large array of transistors. This allows even distribution of the heat generated within each transistor (self-heating) over a sufficient area such that excessive heating does not degrade the performance or reliability of the amplifier.
Under ideal conditions, the current is distributed equally through the many matched HBTs, thus preventing excessive localized self-heating within individual transistors. Unfortunately, traditional wafer fabrication processes have drawbacks, and there can be mismatches between individual HBTs. If the HBTs are mismatched, one HBT will draw a higher collector current and operate at a higher temperature than the other(s). The high collector current increases the junction power dissipation. This causes the junction temperature to rise, which further increases the collector current. Unless properly constructed, this phenomenon of xe2x80x9cthermal runawayxe2x80x9d can cause undesirable permanent damage to the transistor and, therefore, the amplifier.
Thermal runaway results when one HBT fails, causing a chain reaction failure of other components. Unfortunately, small differences in device construction or placement within the IC can cause uneven heating between individual devices. Any device that is connected in parallel with other similar devices and that is hotter than its neighbors will tend to draw more current and heat itself even more, The heating compounds, and the result is a thermal runaway phenomenon that will destroy the HBTs and eventually the amplifier.
Previous attempts to avoid thermal runaway use a single resistance in series with the emitter of each transistor. Usually, the ballast resistances degenerate the DC and RF gain of the transistors such that the increased collector current tends to increase the emitter voltage and thus decreases the emitter-base bias voltage. This reduces the collector current. Typically, thermal stability may be improved by making the ballast resistances large, however, this degrades the RF gain and efficiency of the amplifier. In order to achieve thermal stability, another element, such as an isolation capacitor, may be placed in series with the amplifier. It may be desired that as much power from the DC power supply as possible be transformed into RF power at the output of the amplifier. Any power consumed by the amplifier is wasted and decreases its efficiency.
In addition, the DC and alternating current (xe2x80x9cACxe2x80x9d) collector current that drives the amplifier""s output passes through the ballast resistances. Therefore, significant power is dissipated in the ballast resistances. Because this amplifier is typically fabricated using gallium arsenide (xe2x80x9cGaAsxe2x80x9d), which is a poor heat conductor, the ballast resistances are typically quite large. Unfortunately, a large ballast resistance degrades the efficiency.
Use of a single resistance in series with the base of each transistor of the amplifier may be used in an attempt to avoid thermal runaway. However, making the base ballast resistance large enough to prevent thermal runaway may result in significant power gain degradation. A small value for the base ballast resistance combined with operating at a slightly higher temperature results in each transistor operating at a higher junction temperature and subsequently higher base current, Ib, and collector current, Ic. The physics of the semiconductor junction indicate that, at a higher operating temperature, a lower base to emitter voltage, VBE, is required to produce the same base current. Lowering the value of the base ballast resistance does not allow a sufficient voltage drop between the DC bias voltage and VBE to achieve thermal equilibrium. As a result, this higher voltage causes Ib and Ic to increase, power dissipation to increase, and temperature to increase. As the temperature continues to increase, and VBE is not reduced enough to achieve equilibrium, the process continues until the transistor is destroyed along with the surrounding circuitry. Thus, there is a need for a thermally stable strongly non-linear transistor circuit.
The invention provides a non-linear transistor circuit that has improved thermal stability and improved power efficiency. This circuit includes elements that maintain a constant current to the transistor""s base. In particular, the invention provides a power amplifier that uses separate RF biasing and DC biasing to improve the power efficiency while preventing thermal runaway.
The improved amplifier receives a DC bias voltage supply from any of a number of known sources and includes an RF input port that receives an RF signal to be amplified. A plurality of DC biasing networks are connected to the DC bias voltage supply. Each DC biasing network comprises a resistance large enough to provide thermal stability to the amplifier. A plurality of RF biasing networks are connected to the RF input port. Each RF biasing network comprises a resistance and a capacitance connected in series such that the value of the resistance is small enough to maximize the amplifier gain. The capacitance serves to electrically isolate each transistor. Each DC biasing network and each RF biasing network is connected to a corresponding amplifying transistor. Each transistor is then connected to a common output.
In the case of mismatched transistors, if one transistor is operating at a slightly higher junction temperature and has a subsequently higher Ib, a higher voltage drop across the large value of the DC bias resistance, Rb, will result. A point of thermal equilibrium is reached when the voltage drop across the DC bias resistance, Rb, sufficiently reduces the voltage between the transistor""s base and emitter, VBE, hence reducing Ib and Ic, and therefore, sufficiently reducing the junction temperature. The DC bias resistances are large enough such that each of the DC bias resistances thermally stabilizes each of the corresponding transistors.
The DC bias voltage and the RF signal to be amplified are supplied to the amplifier. The DC bias voltage is supplied to the plurality of DC bias resistances, that output a modified DC bias voltage. The RF signal is supplied to the RF bias network, which outputs a modified RF signal. The modified signals are then supplied to each of the corresponding transistors that amplify the modified RF signals. All of the amplified RF signals are then supplied to a common RF output port.
Other systems, methods, features, and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.